The present invention relates, generally, to liquid crystal display assemblies and, more particularly, relates to miniature liquid crystal display assemblies constructed to reduce residual stresses.
In the recent past, substantial research and development resources have been directed toward small scale Liquid Crystal Display (LCD) and light valve technologies. These miniature LCD assemblies are typically employed in high resolution projection displays, such as a reflective LCD projectors, SXGA formats (1,280xc3x971,024 pixel resolution) and even HDTV formats (above 1,000 line resolution), or the like.
Briefly, as shown in FIGS. 1 and 2, a conventional small scale LCD assembly 20 is illustrated including a die 21 having a pixel array 22. This pixel array 22 is typically composed of rows and columns of electrically conductive pathways each forming an individual pixel (not shown). Each pixel can be individually changed to an xe2x80x9conxe2x80x9d condition by selecting the appropriate row and column of pixel array 22. Positioned around or concentrated on one end of the pixel array are a plurality of die bond pads 23 which are internally connected to the pixel array 22 to enable operational control thereof. Selection of the appropriate pixel is controlled by control circuitry, either included within the die 21 or external to the die 21. In either configuration, external control signals may be used to control the functions of the die 21.
As best viewed in FIGS. 2 and 3, a transparent glass plate 24 is typically placed over the die 21 and the pixel array 22, such that a portion of the glass plate 24 overhangs the die 21. The glass plate 24 is, usually affixed to die 21 through an adhesive seal 25 which together cooperate to define a sealed volume encompassing the pixel array 22. This sealed volume is then commonly filled with a solution 26 of Polymer Dispersed Liquid Crystals (PDLC). To facilitate grounding of the glass plate 24, a conductive coating (not shown) may be deposited over the undersurface 28 thereof.
The die 21 is typically rigidly or semi-rigidly mounted to a substrate 27 for mounting support and heat conductive dissipation for the die. A conductive adhesive 29 (FIG. 3), such as a conductive epoxy, is generally applied to the undersurface 28 of the die 21 to affix the die directly to the top surface of the substrate 27. Accordingly, a heat conductive pathway is created directly between the die and the substrate to dissipate heat generated by the die.
The substrate 27 generally includes a plurality of substrate bond pads 30 which are typically wire bonded to the die bond pads 23 through bonding wires 31. Finally, a glob coating 32 is applied to seal die 21 to substrate 27. The glob coating 32 (FIG. 3) further normally encapsulates the bonding wires 31 and the internal elements of die 21 without obscuring a view of the pixel array 22 through the glass plate 24.
By activating the appropriate pixels, the corresponding liquid crystals in the PDLC, deposited in sealed volume, are caused to either align or disperse. Upon alignment, light is permitted to pass through the aligned crystals and the adjacent glass plate, thus appearing light in color. In contrast, when the liquid crystals are dispersed, light is prevented from passing therethrough and, hence the glass plate 24, so that the corresponding pixel appears dark in color.
One important aspect in the proper operation of these small scale LCD or light valve assemblies is the maintenance of proper distance uniformity (preferably about 2-4 xcexcm) between the pixel array and the undersurface 33 of the glass plate. Variances in the separation of the glass plates may often times cause the pixel array to function improperly or cause operational failure.
Conventional rigid display device constructions, for example, often warp during operation since the substrate 27, the glass plate 24 and the silicon die 21 are all composed of materials or composites having different coefficients of expansion. The individual components of the LCD assembly, therefore, often expand at different degrees and rates. Further, depending in part upon the construction processes, such as the adhesive curing techniques, significant residual stresses may be induced upon the cell. Eventually, in severe instances, the glass plate 24 may delaminate from the die 21. At a minimum, these internal stresses cause optical defects such as variations in color uniformity and fringes, and variations in the cell gap thickness may cause optical shadows.
This is especially true since the undersurface 28 of the die 21 is typically rigidly affixed or attached directly to the substrate. For example, when the substrate and the dies are both composed of a silicon material, upon heating, the glass plate expansion tends to negatively bow or warp (FIG. 4) at a rate greater than that of the die and substrate. Therefore, upon more extensive high temperature thermal cycling during operation, the glass plate 24 may eventually delaminate from the die 21 to expose the Polymer Dispersed Liquid Crystals (PDLC) 26.
In contrast, when the die 21 is composed of a silicon material and the substrate 27 is composed of a more conductive material, such as aluminum, upon heating, the substrate expansion tends to positively bow or warp (FIG. 5) the substrate at a rate greater than that of the die 21 and glass plate 24. As viewed in the cross-sectional view of FIG. 5, central thinning of the PDLC 26 is caused which result in discoloration and the appearance of optical shadows and nonuniformity.
Moreover, during low temperature conditioning, the glass plate 24 often fractures due to internal stress induced by the substrate, which is then transmitted to the glass through the rigidly mounted die. This is especially problemsome at the regions where the adhesive mounts the die to the substrate, and/or where the glob coating contacts the glass.
Accordingly, there is a need to provide a LCD assembly which minimizes residual stress induced upon the cell.
The present invention provides a liquid crystal display assembly comprising a display device and a support substrate. The display device includes a die having a pixel array, and a transparent plate positioned over the die. An adhesive seal adhesively couples the die to the transparent plate, which together with the transparent plate and the die cooperate to define a sealed volume therebetween encompassing the pixel array. A liquid crystal material is disposed within the sealed volume. The support substrate is coupled to the transparent plate for support of the display device such that the die is substantially insulated from transmission of residual stresses induced by or acting upon the substrate.
In one aspect of the present invention, the substrate may be mounted to the transparent plate at a single point location, while in another aspect, the substrate may be mounted to the transparent plate proximate a single peripheral edge portion thereof. An adhesive may be positioned between the substrate and the transparent plate for either single point mounting or the single peripheral edge portion mounting.
In another aspect of the present invention, the substrate provides a cavity formed and dimensioned for receipt of the die therein which is of a depth sufficient for non-contact between the die undersurface and the substrate.
In still another aspect of the present invention, a liquid crystal display assembly includes a die having a pixel array, a top surface and an opposite undersurface. A transparent plate is provided coupled to the die through an adhesive seal. The adhesive seal, the transparent plate and the die cooperate to define a sealed volume therebetween encompassing the pixel array, which contains a liquid crystal material disposed therein. A support substrate is coupled to the transparent plate for support of both the transparent plate and the mounted die thereon in a manner substantially non-attached to the die.
The substrate defines a cavity in a top surface thereof formed and dimensioned for receipt of the die therein, and is of a depth sufficient for substantial non-contact between the die undersurface and the substrate such that a gap is formed between a floor of the cavity and the undersurface of the die. A thermoconductive material may be positioned in the cavity between and contacting the die undersurface and the cavity floor for heat conduction therebetween.
In yet another aspect of the present invention, a liquid crystal display assembly is provided including a die having a pixel array, a top surface and an opposite undersurface, and a transparent plate. An adhesive seal adhesively couples the die to the transparent plate such that a ledge portion thereof extends beyond a peripheral edge portion of the die. Similarly, the adhesive seal, the transparent plate and the top surface of the die cooperate to define a sealed volume encompassing the pixel array in which a liquid crystal material is deposited therein. A relatively rigid support substrate includes a shoulder portion adapted to cooperate with the ledge portion of the transparent plate for support of the transparent plate and the die with the substrate while the underside surface of the die remains relatively free of substantial supportive mounting to the substrate. A conductive adhesive mounting the plate ledge portion to the substrate shoulder portion.
In another aspect of the present invention, a method of packaging a liquid crystal display assembly is included comprising the steps of: providing a display device including a die having a pixel array, a transparent plate, an adhesive seal adhesively coupling the die to the transparent plate, and a liquid crystal material disposed within a sealed volume formed between the adhesive seal, the transparent plate and the die; and mounting the transparent plate to a support substrate for support of the display device such that the die is substantially insulated from transmission of residual stresses induced by or acting upon the substrate.
The mounting step may include the step of applying an adhesive to not more than a single point location or not more than a single strip location between the transparent plate and the substrate for mounting thereto. In another embodiment, the present inventive method includes the step of providing a cavity in a top surface of the substrate formed and dimensioned for receipt of the die therein when the display device is mounted to the substrate.
In another method of the present invention, a method of packaging a liquid crystal display assembly is included comprising the steps of: adhesively coupling a transparent plate to a die having a pixel array such that a ledge portion of the plate extends beyond a peripheral edge portion of the die; and depositing a liquid crystal material within a sealed volume formed between the adhesive seal, the transparent plate and the die. The inventive method further includes the step of affixing the ledge portion of the transparent plate to an upstanding shoulder portion of a support substrate for support of the transparent plate and the die with the substrate while an underside surface of the die remains relatively free of substantial supportive mounting to the substrate.